1. Technical Field
The present invention relates to an apparatus for data storage in general, and in particular, to an apparatus for storing data within a data processing system. Still more particularly, the present invention relates to a direct access storage device having multiple track density for storing data within a data processing system.
2. Description of the Prior Art
Direct access storage devices (DASDs) are generally utilized for long-term storage of data within a data processing system, and hard disk drives are one type of DASDs. A hard disk drive utilizes several rotatable disks for data storage. Data are represented on the disks as a series of magnetically polarized regions. These polarized regions are arrayed along the surface of a disk as either concentric data tracks or spiral data tracks. A read/write head is utilized to read data from or write data to different data tracks on the disk while the disk is rotating underneath the read/write head.
Increasing usage of high-volume file servers in local-area networks and wide-area networks has led to an increasing demand for higher capacity hard disk drives. Thus, dramatic improvements on the hard disk storage technology are required in order to keep pace with such demand. Generally, the storage capacity of a hard disk drive can be expanded by increasing both the track density and the linear data density of the hard disk drive. In conjunction, the track-seeking speed of the hard disk drive should also be increased accordingly.
In order to assure performance and reliability for a high track density design, the read/write head of the hard disk drive must be positioned extremely accurately over the data track center by a servo controller. At the same time, because high track-seeking speed tends to cause poor head settling and high track-misregistration (TMR) to the hard disk drive, the servo controller must also be optimized to provide a minimal Seek-Settling TMR after moving the head from one track to another and to maintain an adequate Tracking TMR during read and write operations.
Nevertheless, although a high bandwidth servo controller designed to have an aggressive seek profile can perform seeking with a very short seek-time, a fast seek-time often induces extra head vibrations and instability that lead to a high Seek-Settling TMR. For example, a typical fast-seek operation across one-third of a disk data surface for a 3.5-inch drive can complete in about 8 ms, but such high seek-time could also cause a Seek-Settling TMR as high as 10% of a full track width. Certainly, a low bandwidth servo controller having a less aggressive seek profile (e.g., seek time&gt;13 ms) can reduce the Seek-Settling TMR significantly, if not eliminate the Seek-Settling TMR completely. Further, a low bandwidth servo controller can even substantially improve the Tracking TMR during the read and write operations because of the added attenuation on the dynamic noise, resonances, servo-track format errors, and position-sensing errors. However, the penalty is that all read and write operations will also be delayed by the low bandwidth servo controller due to the prolonged seeking operations.
This is the dilemma: it is possible to increase the storage capacity of a hard disk drive without increasing the TMR simply by utilizing a low bandwidth servo controller; however, the resulting degraded performance from the prolonged seeking operation is undesirable for any normal user applications. On the other hand, a higher TMR may be acceptable for a higher storage capacity design with an improved error correction coding (ECC) and processing. At least the addition of extra ECC in the data storage can improve the soft error rate (SER). For example, by doubling the ECC from 20 bytes to 40 bytes in a data block of 512 bytes, statistically, SER is improved from 10e.sup.-10 to 10e.sup.-20. Nonetheless, the additional ECC and processing also decrease the data access performance which again is undesirable for normal user applications.
Consequently, it would be desirable to provide an improved direct access storage device with a high data-storage capacity but without the degraded data-access performance.